Spark suppressing brush brush holder assembly for rotating machines and sliding contacts

ABSTRACT

The inclusion of a body of polycrystalline metal oxide varistor material in a brush or brush holder for the suppression of sparking between sliding contacts is disclosed. The body of metal oxide varistor material may form part of a brush contact at a leading or trailing edge thereof or may be the material comprising all, or part of a brush holder in slidable contact with the brush contact and a second contact. Accordingly, the initial electrical connection and, or the final electrical connection, between the brush contact and the other slidable contact is through the varistor material thereby reducing sparking as the sliding contacts make and break connection.

Unite States Patent 1 Shattuck et al.

[ June 26, 1973 1 1 SPARK-SUPPRESSING BRUSH-BRUSH HOLDER ASSEMBLY FORROTATING MACHINES AND SLIDING CONTACTS [73] Assignee: General ElectricCompany,

Schenectady, N.Y.

[22] Filed: Sept. 10, 1971 [21] App]. No.: 176,852

[52] US. Cl. 310/220, 310/239 [51] Int. Cl. H02k 13/06 [58] Field ofSearch 310/220, 221, 222,

[56] References Cited UNITED STATES PATENTS 3,509,398 4/1970 Treible310/220 2,274,592 2/1942 Dearborn 264/104 3,052,764 9/1962 De Rugeris310/248 2,470,096 5/1949 Eck 310/220 UX 3,630,970 12/1971 3,507,9384/1970 l-lundsdiecker 3,146,365 8/1964 Millar 310/240 3,466,481 9/1969Sckerl 310/239 Primary ExaminerR. Skudy Attorney-John F. Ahern et a1.

[5 7] ABSTRACT The inclusion of a body of polycrystalline metal oxidevaristor material in a brush or brush holder for the suppression ofsparking between sliding contacts is disclosed. The body of metal oxidevaristor material may form part of a brush contact at a leading ortrailing edge thereof or may be the material comprising all, or part ofa brush holder in slidable contact with the brush contact and a secondcontact. Accordingly, the initial electrical connection and, or thefinal electrical connection, between the brush contact and the otherslidable contact is through the varistor material thereby reducingsparking as the sliding contacts make and break connection.

29 Claims, 6 Drawing Figures PAIENIEDJUNZG ms 3.742.272

' I IIHTW IIIIIIH ||||1||| I 0.0! OJ LO I0 I00 AMPERES PER UNIT AREAINVENTOR DONALD P. SHATTUCK JOHN D. HARNDEN, JR.

BY KM SPARK-SUPPRESSING BRUSH-BRUSH HOLDER ASSEMBLY FOR ROTATINGMACHINES AND SLIDING CONTACTS Our invention relates to an improvedelectric current collection means utilizing a body of metal oxidevaristor material, and in particular, to a brush-brush holder assemblywherein the brush or brush holder is comprised in part by such metaloxide varistor mateiral in the'current path between the brush andcommutator segment upon parting of the commutator segment from the brushwhereby the nonlinear resistance characteristics of the metal oxidevaristor material causes substantially reduced sparking.

The brushes in rotating brush-type machines and the like are generallyfabricated of carbon, a relatively poor electrical conductor, reinforcedby other materials. The brushes are retained in position by brushholders which generally are in the form of square or rectangular sleevesserving as a guide for any radial motion of the brush resulting fromvibration or eccentricity of the armature. The brush holder may bemounted on a bracket to maintain a rigid position spaced from thecommutator surface. An adjustable spring connected to the bracket bearson the top surface of the brush to maintain a desired contact pressureof the bottom surface of the brush upon the commutator segments.

The electrical current which is commutated in various rotatingelectrical brush-type machines such as a universal motor and directcurrent motor, and in apparatus such as a variable autotransformer, isinductive in nature and therefor tends to produce transient voltagesbetween the commutator segment and brush when the two part, resulting insparking. The sparking results in excessive brush wear, commutator bardeterioration and radio interference among other disadvantages, and canlead to flashovers to high current levels all of which limit machineperformance. The prior art has been directed to various improvements indesign of rotating machines in order to reduce brush-commutator sparkingthrough different types of brushes, the use of commutating poles andelectronic commutating circuits, and increased number of commutator barsor segments but the sparking is still often considered to be excessive.

Therefore, one of the principal objects of our invenwhere V is thevoltage between two points separated by a body of the material underconsidertion, I is the current flowing between the two points, C is aconstant and a is an exponent greater and 1. Both C and a are functionsof the geometry of the bodyformed from the material and the compositionthereof, and C is primarily a function of the material grain sizewhereas a is primarily a function of the grain boundary. Materials suchas silicon carbide exhibit nonlinear or exponential resistancecharacteristics and have been utilized in commercial silicon carbidevaristors, however, such nonmetallic varistors typically exhibit an aexponent of no more than 6.

A new family of varistor materials having alphas in excess of 10 withinthe current density range of 10 to 10 amperes per square centimeter hasrecently been produced from metal oxides although very few applicationshave been found for this new metal oxide varistor material, alsoreferred to herein as MOV, a trademark of the General Electric Company.Although the alpha of the MOV materials in which range the alpha remainssubstantially constant, are identified by the current desnity range of10' to 10 amperes per square centimeter, it is appreciated that thealphas remain high also at higher and lower currents although somedeviation from maximum alpha values may occur. The MOV material is apolycrystalline ceramic material formed of a particular metal oxide withsmall quantities of one or more other metal oxides being added. As oneexample, the predeominant metal oxide is zinc oxide with smallquantities of bismuth oxide being added. Other additives may be aluminumoxide, iron oxide, magnesium oxide, and calcium oxide as other examples.The predominant metal oxide is sintered with the additive oxide(s) toform a sintered ceramic metal oxide body. Since the MOV varistor isfabricated as a ceramic powder, the MOV material can be pressed into avariety of shapes of various sizes. Being polycrystalline, thecharacteristics of the MOV varistor are determined by the grain(crystal) size, grain composition, grain boundary composition, and grainboundary thickness, all of which can be controlled in the ceramicfabrication process.

The nonlinear resistance relationship of the MOV is such that theresistance is very high (up to approximately 10,000 megohms) at very lowcurrent levels in the mircoampere range and progresses in a nonlinearmanner to an extremely low value (tenths of an ohm) at high currentlevels. The resistance is also more nonlinear with increasing values ofalpha. These nonlinear resistance characteristics result in voltageversus current characteristics wherein the voltage is effectivelylimited, the voltage limiting or clamping action being more enhanced atthe higher values of the alpha exponent as shown in FIG. 1. Thus, thevoltage versus current characteristics of the MOV is similar to that ofthe Zener diode with the added characteristic of being symmetricallybidirectional and over more decades of current. The breakdown mechanismof the MOV is not yet clearly understood but is completely unlike theavalanche mechanism associated with Zener diodes, a possible theoreticalexplanation of its operation being that of space charge limited current.The breakdown voltage of an MOV device is determined by the particularcomposition of the MOV materiaal and the thickness to which it ispressed in the fabrication process. The MOV involves conduction changesat grain boundaries resulting in the advantage of bulk phenomenonallowing great flexibility in the design for specific applicationssimply by changing the dimensions of the body of MOV material. That is,the current conduction in the absence of closely spaced electrodes alongone surface of the MOV body is through the bulk thereof. The bulkproperty of the MOV also permits a much higher energy handlingcapability as compared to junction devices. Thus, since an MOV devicecan be built up to any desired thickness, it is operable at much higher7 voltages than the Zener diode junction device and can be used in arange from a few volts to severals kilovolts. The voltage changes acrossa silicon carbide varistor device are much greater than across an MOVdevice for a given current change and thus the silicon carbide varistorhas a much smaller voltage operating range thereby limiting itsapplications. The thermal conductivity of MOV material is fairly high(approximately one-half that of alumina) whereby it has a much higherpower handling capability than silicon carbide, and it exhibits anegligible switching time in that its response time is in thesubnanosecond domain. Finally, the MOV material and devices made thereofcan be accurately machined, soldred, and operated at very low voltages,capabilities not possible for the larger grained silicon carbide.

Therefore, another principal object of our invention is to provide abody of metal oxide varistor material in the current path between thebrush and the commutator segment of a rotating electrical machine whenthey part to obtain the reduced spark suppression.

A further object of our invention is to form at least one portion of thebrush of the metal oxide varistor material.

A still further object of our invention is to form at least a portion ofthe brush holder of the metal oxide varistor material.

In accordance with our invention, we provide an improved brush-brushholder assembly wherein a body of metal oxide varistor material havingan alpha exponent in excess of 10 is included in the current pathbetween the brush and a segment of the commutator when they part. Thenonlinear resistance characteristic of the metal oxide varistor materiallimits the voltage build-up across the parting segment and brush andthereby pro vides superior spark-suppression during operation of therotating machine.

The body of metal oxide varistor material can be integrated into thebrush along the leading and, or trailing edges thereof as one embodimentof our invention. Such material can also be incorporated into the brushholder as the whole or merely bottom portion thereof bearing against thecommutator segments to thereby enhance the transfer of current betweenbrush and commutator segments with reduced sparking.

The features of our invention which we desire to protect herein arepointed out with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation,together with further objects and advantages thereof may best beunderstood by reference to the following description takenin connectionwith the accompanying drawings wherein like parts in each of the severalfigures are identified by the same reference character and wherein:

FHG. l is a graphical representation of the nonlinear resistance andresultant voltage limiting characteristics of the MOV material fordifferent values of the exponent alpha plotted in terms of volts vs.amperes on a log-log scale;

FIG. 2 is an elevation view, partly in section, of a brush-brush holderassembly in accordance with our invention wherein the brush isfabricated with the MOV meterial along its leading and trailing edges;

FIG. 3 is an elevation view, partly in section, of a brush-brush holderassembly wherein bodies of MOV material are located between the brushholder and brush along both the leading and trailing edges;

FIG. 4 is an elevation view, partly in section, indicating an entirebrush holder fabricated of the MOV material;

FIG. 5 is an elevation view, partly in section, indicating the bottomportion of the brush holder fabricated of the MOV material; and

FIG. 6 is an elevation view, partly in section, indicating a body of MOVmaterial attached along the trailing edge of the brush holder.

The volts versus amperes characteristics plotted in FIG. 1 of thedrawings illustrate the nonlinear or exponential resistancecharacteristics exhibited by MOV material, and in particular, indicatethe increasing nonlinearity and enhanced voltage limiting obtained withincreased values of the exponent a. The VOLTS abscissa'is in terms ofthe brush-to-commutator segment voltage drop and the CURRENT ordinate isin terms of current or current density. Although the use of linearscales on the graph would show the decreasing slopes (decreasingresistance values) with increasing currents, such curves can be readilymanipulated by the choice of scales, and for this reason, log-log scalesare chosen to obtain a family of lines each of which remainssubstantially straight within the indicated current range. It can beseen from the FIG. 1 plots that the resistance exhibited by the MOVmaterial is quite high at low current levels and becomes increasinglysmaller in a nonlinear exponential manner with increasing currentlevels, and such nonlinearity is greater for greater values of theexponent alpha. Extension of the to lower plots higher current levelswould obviously indicate correspondingly much higher and lowerresistances, respectively, and operation of the subject machines maytransiently reach such levels. The leakage current through the MOVmaterial is negligible compared to the rated machine current duringnormal operating conditions.

Referring now to PEG. 2, there is shown an improved brush-brush holderassembly constructed in accordance with our invention wherein the MOVmaterial is incorporated in selected portions of the brush. The brush,illustrated as a whole by numeral It) may comprise a conventional carbonbrush but having leading and trailing edge portions thereof beingfabricated of the hereinabove described MOV material. The MOV portions,ill, 12 may be of rectangular form as illustrated, as one example of themany shapes of such MOV portions. The MOV bodies ill and T2 are suitablybonded to the main body of the brush Mi such that the entire brush bodyforms the conventional generally rectangular shape associated withbrushes. The bonding may be accomplished by low temperature soldering orby pressure contact as two examples, in both cases a metallized surfacebeing utilized on the MOV material. The metallized surface may beobtained, for example, by firing a thin layer of glass and silverparticles on the MOV surface. Ohmic contact is utilized, in general, inorder to take advantage of the bulk phenomenon operation of the MOVmaterial. The bottom surfaces of the MOV bodies 11 ii and t2 areslightly curved and flush with the bottom surface of brush M which is incontact with the upper surfaces of the electrically conductive segmentsR3 of the commutator.

The brush MD is in contact with the surface of the commutator (i.e.,rides on the surface thereof) due to the force of a spring applied bybrush spring 14 against the top surface of brush 10. Brush is maintainedin its desired position relative to the commutator by means of brushholder 15 which is held in a fixed position spaced from the surface ofthe commutator by means of bracket 16. Brush holder 15 is ofconventional design comprising a rectangular sleeve fabricated of a highstrength metal and serves as a guide for any radial motion of the brush10 resulting from vibration or eccentricity of the armature orcommutator. For purposes of simplicity, the flexible copper cablegenerally described as a brush shunt or pigtail which is often employedfor directing the current from the brush to the brush holder is notillustrated but can be employed, as desired.

The MOV material preferably comprises lO-3O percent of the volume of thebrush 10 although it should be evident that lesser and greater amountsof such MOV material may be utilized as required by particularapplications without departing from the scope of our invention. Theparticular choice of the percentage of MOV material will depend on theoptimal choice of number of commutator bars or segments which may bereduced as a result of our invention and the corresponding need forgreater energy absorption in the MOV material. In the case wherein themachine rotates only in a single direction, it may be sufficient toprovide the MOV material along only the trailing edge of the brush 10.The MOV material by being located in at least the trailing edge of thebrush is therefore in the current path of the brush when such brush andcommutator segment first part upon rotation of the commutator. The highthermal conductivity of the MOV material also permits cooler operationof the brushes.

The principal advantage of our invention is, of course, the superiorsuppression of sparks and arcs which tend to develop particularlybetween the trailing edgs of the brushes and the commutator segments, oralternatively, to permit construction of a machine having fewercommutator segments and corresponding higher speed and lower costs. Thissuperior suppression is obtained primarily due to the following threeexceptional properties of MOV material (I) the resistancecharacteristics are highly nonlinear (a 10) over a very wide range ofcurrent and result in a high degree of voltage limiting, (2) theresponse time is negliglble, and (3) the high thermal conductivitypermits rapid dissipation of heat developed in the commutator-brushspark generating phenomenon. In conventional commutation, upon initialparting of the commutator segment and brush, the current cannot changeinstantaneously and the inductive voltage e L (di/dt) must increase toforce the same current flow to continue thereby creating the sparkingand possible resulting arcing. However, the use of our body of MOVmaterial limits the voltage build-up across the parting segment andbrush and provides a relatively low resistance path for the currentwhich thence decays at a rate determiuned primarily by .the LR timeconstant of the machine or until a current zero is reached, theresistance of the MOV body increasing substantially as the voltage, andprimarily the current, are decreasing. That is, the body of MOV materialremains in the path of the current after the brush and commutatorsegment have first parted due to the location and geometry of such MOVbody, and since the path of the current increases as the segment movesaway from under the brush, the brushto-segment voltage drop slowlyincreases and rapidly dissipates the stored inductive energy until, asthe segment leaves the MOV body, the energy has reached zero. The amountof MOV material in the brush, and in particular, the ratio of the bottomsurface of the MOVmaterial to the bottom surface of the entire brush ischosen such that at maximum machine speed there is no current at theseparation time of the body of MOV material and segment. Thus,commutator-brush sparking is substantially eliminated, or at leastsignificantly reduced with our brush as compared to conventionalbrushes.-

FIG. 3 illustrates an embodiment of our invention wherein two slabs ofthe MOV material 20 and 21 are inserted in the brush holder and retainedtherein between the inner end surfaces thereof and the outer leading andtrailing end surfaces of a conventional carbon brush 10. The MOV bodies20 and 21 are spring loaded by means of spring 22 independently of thespring loading of brush 10 as provided by spring 14 bearing on the brushtop surface. Bodies 20 and 21 may be each of substantially rectangularform and, as mentioned with reference to the FIG. 2 embodiment, it wouldbe adequate to utilize only a single scuh body along the trailing edgeof the brush for single direction rotating machines. The advantage ofthe FIG. 3 embodiment is that the separate spring loading of the MOVbody and the brush provides compensation for differences in wear rate ofthe MOV mateirial and brush material as well as for the thermalexpansion and contractions thereof.

A third embodiment of our invention is illustrated in FIG. 4 wherein theentire brush holder 15 is fabricated of the MOV material and a spring 17forces the bottom surface of brush holder 15 to maintain a desiredpressure contact with the rotating commutator. The brush 10 is of theconventional type in this particular embodiment.

FIG. 5 illustrates a fourth embodiment of our invention which is verysimilar to the embodiment illustrated in FIG. 4 except that the MOVmaterial 15a is located only along the bottom portion of the brushholder 15 since this is the optimum position of such material to enhancecommutation. The brush holder 15 is also in contact with the rotatingcommutator as in the case of the FIG. 4 embodiment although it isevident that only the MOV portion thereof is in contact with thecommutator. The MOV portion 15a of the brush holder is rigidly fastenedto the main body 15 of the brush holder by any convenient means such asbonding.

A fifth embodiment of our invention is illustrated in FIG. 6 wherein aconventional carbon brush 10 is utilized and a body of the MOV materialis mounted along the trailing end of a conventional brush holder 15 andextends beyond the bottom surface thereof. Thus, a generally rectangularbody of MOV material 1512 may be rigidly attached to the inner or outersurface of the trailing end of the brush holder, or alternatively, mayform a trailing end thereof by being suitably connected across theadjacent two side walls of the brush holder. The body of MOV materialmay be located only along the lower portions of the trailing end of thebrush holder and when forming the trailing end thereof may also be onlyalong the lower portion thereof, the upper end portion remote from thecommutator being fabricated of the conventional brush holder materialwhich forms the other three sides. In the case of a reversible machine,the MOV bodies 15b are located along both the leading and trailing edgesof the brush holder. Spring 117 causes the bottom surface of MOV body15b, but not the remaining bottom surface of the brush holder, to bearagainst the commutator and body llb is thus spring loaded independnetlyof a brush it). In all of the above applications, the MOV body isselected for a normal operating voltage just slightly above the expectedcommutator segment-to-segment (or brush) voltage drop.

Having described five embodiments of our improved brush-brush holderassembly, it should be obvious that the bodies of MOV material canassume any of a number of shapes as dictated by the particularapplications and may be positioned somewhat differently from thatillustrated in the drawings, the criteria being that such body or bodiesof MOV material be located in the resulting path of the current betweenthe brush and commutator segment upon parting thereof. Although only onebrush is shown in each of FIGS. 2-6, it should be apparent that aconventional parallel array of separted brushes may also be retained ina brush holder, especially in the large size machines. The brush-brushholder and geometry of such assemblies can also be of other forms thanthat illustrated, as commonly used in smaller electric machines such asvacuum cleaners, mixers, hand tools, and the like, and the invention canalso be utilized with nonrotating machines such as a tap changer orbrush shifters in variable autotransformers. in this latter case, abrush having a flat bottom surface contacts flattened areas of aconductor or conductors wound on a magnetic body and the brush is movedin a circular are. Our invention thus is broadly directed to improvedelectric current collection means wherein two electrically conductivemembers are adapted to move relative to each other and are in closeproximity to each other for achieving electric current transfertherebetween. A body of the metal oxide varistor material is disposed inconjunction with the current path between the two members and results inimprovement of the current transfer, and, or improved commutation of thecurrent. The invention is also applicable in this general area ofelectric current collection as employed in various types of slip-rings,third-rail collectors, and trolley wire-panograph current collectors.Our invention thus introduces a new freedom of design in brushtypemachines whereby the normal commutation barto-bar voltage restriction isalleviated thus permitting realization of machine design with higheroperating speed, increased surge-current handling capability, simplifiedmachine construction and maintenance, and greater economic freedom.

Thus, while our invention has been particularly shown and described withreference to five illustrated embodiments thereof, it should beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the scope of theinvention as defined by the following claims.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

ll. An improved spark-suppressing brush-brush holder assembly forrotating electrical brush-type machines and the like comprising aspring-loaded brush,

a brush holder for retaining said brush in a generally fixed positionrelative to a rotating commutator against which said brush maintains acontact pressure determined by the spring-loading thereof, and a body ofmetal oxide varistor material forming a portion of said brush along thetrailing edge thereof in contact with the commutator disposed in thecurrent path between said brush and segment of the commutator when thebrush and commutator segment part during rotation of the commutator, thebody of metal oxide varistor material having nonlinear resistancecharacteristics which limit the voltage developed across the commutatorsegment and brush when they first part. 2. The improved brush-brushholder assembly set forth in claim 1 wherein said body of metal oxidevaristor material is a first body forming a portion of said brush alongthe trailing edge thereof in contact with the commutator and a secondbody forming a portion of said brush along the leading edge thereof incontact with the commutator. 3. The improved brush-brush holder assemblyset forth in claim 1 wherein said body of metal oxide varistor materialcomprises a first body spring loaded independently of the spring loadingof said brush and retained between the trailing end surface of saidbrush and an adjacent inner end surface of said brush holder, theindepenent spring loading of said brush and said body of metal oxidevaristor material providing compensation for differences in wear rate ofthe metal oxide varistor material and brush material due to contact withthe rotating commutator. 4. The improved brush-brush holder assembly setforth in claim 3 and further comprising a second body of metal oxidevaristor mateiral spring loaded independently of the spring loading ofsaid brush and retained between the leading end surface of said brushand an adjacent inner end surface of said brush holder. The improvedbrush-brush holder assembly set forth in claim 3 wherein said first bodyof metal oxide varistor material is of a generally rectangular shape. 6.The improved brush-brush holder assembly set forth in claim 1 whereinsaid brush holder is fabricated of said body of metal oxide varistormaterial, and including spring means in communication with said brushholder for maintaining a contact pressure of the bottom surface of saidbrush holder upon the commutator. '7. The improved brush-brush holderassembly set forth in claim 1 wherein a bottom portion of said brushholder is fabricated of said body of metal oxide varistor material, andincluding spring means in communication with said brush holder formaintaining a contact pressure of the bottom surface of said brushholder upon the commutator.

8. The improved brush-brush holder assembly set forth in claim Itwherein a bottom portion of said brush holder along the trailing endthereof is fabricated of said body of metal oxide varistor material andextends beyond the bottom surface of the remainder of said brush holder,and including spring means in communication with said brush holder formaintaining a contact pressure of the bottom surface of said body ofmetal oxide varistor material upon the commutator.

9. The improved brush-brush holder assembly set forth in claim 1 whereinsaid body of metal oxide varistor material is rigidly attached to an endsurface of said brush holder along the trailing edge thereof and extendsbeyond the bottom surface of said brush holder, and including springmeans in communication with said brush holder for maintaining a contactpressure of the bottom surface of said body of metal oxide varistormaterial upon the commutator.

10. The improved brush-brush holder assembly set forth in claim 1wherein said body of metal oxide varistor material is connected acrosstwo side walls of said brush holder to form the trailing end thereof,said body of metal oxide varistor material extending beyond the bottomsurface of said brush holder, and including spring means incommunication with said brush holder for maintaining a contact pressureof the bottom surface of said body of metal oxide varistor material uponthe commutator.

11. The improved brush-brush holder assembly set forth in claim 1wherein said body of metal oxide varistor material has an alpha exponentin excess of 10.

12. An improved spark-suppressing brush-brush holder assembly forbrush-type electrical machines comprising a spring-loaded brush,

a brush holder for retaining said brush in a desired position relativeto one or more electrical conductors against which said brush maintainsa contact pressure determined by the spring-loading thereof, and

a body of metal oxide varistor material forming a portion of said brushalong the trailing edge thereof in contact with one of said conductorsdisposed in the current path between said brush and said conductors whenthe brush and said conductors part during relative motion between saidconductors and brush-brush holder assembly, the body of metal oxidevaristor mateiral having nonlinear resistance characteristics whichlimit the voltage developed across the brush and each conductor whenthey first part.

13. The improved brush-brush holder assembly set forth in claim 12wherein said body of metal oxide varistor material is a first bodyforming a portion of said brush along the trailing edge thereof and asecond body forming a portion of said brush along the leading edgethereof.

14. The improved brush-brush holder assembly set forth in claim 12wherein said body of metal oxide varistor material comprises 6 a firstbody spring loaded independently of the spring loading of said brush andretained between the trailing end surface of said brush and an adjacentinner end s'urface of said brush holder, the independent spring loadingof said brush and said body of metal oxide varistor material providingcompensation for differences in wear rate of the metal oxide varistormaterial and brush material due to the relative motion between saidconductors and brush-brush holder assembly. 15. The improved brush-brushholder assembly set forth in claim 14 and further comprising a secondbody of metal oxide varistor material spring loaded independently of thespring loading of said brush and retained between the leading endsurface of said brush and an adjacent inner end surface of said brushholder. 16. The improved brush-brush holder assembly set forth in claim14 wherein said first body of metal oxide varistor material is of agenerally rectangular shape. 17. The improved brush-brush assembly setforth in claim 12 wherein said brush holder is fabricated of said bodyof metal oxide varistor material, and including spring means incommunication with said brush holder for maintaining a desired contactpressure of the bottom surface of said brush holder upon the conductors.18. The improved brush-brush holder assembly set forth in claim 12wherein a bottom portion of said brush holder is fabricated of said bodyof metal oxide varistor material, and including spring means incommunication with said brush holder for maintaining a contact pressureof the bottom surface of said brush holder upon the conductors. 19. Theimrpoved brush-brush holder assembly set forth in claim 12 wherein abottom portion of said brush holder along the trailing end thereof isfabricated of said body of metal oxide varistor material and extendsbeyond the bottorn surface of the remainder of said brush holder, andspring means in communication with said brush holder for maintaining acontact pressure of the bottom surface of said body of metal oxidevaristor material upon the conductors. 20. The improved brush-brushholder assembly set forth in claim 12 wherein said body of metal oxidevaristor material is rigidly attached to an end surface of said brushholder along the trailing edge thereof and extends beyond the bottomsurface of said brush holder, and spring means in communication withsaid brush holder for maintaining a contact pressure of the bottomsurface of said body of metal oxide varistor material upon theconductors. 21. The improved brush-brush holder assembly set forth inclaim 12 wherein said body of metal oxide varistor material is connectedacross two side walls of said brush holder to form the trailing endthereof, said body of metal oxide varistor material extending beyond thebottom surface of said brush holder, and spring means in communicationwith said brush holder for maintaining a contact pressure of the bottomsurface of said body of metal oxide varistor material upon theconductors. 65 22. The improved brush-brush holder assembly set forth inclaim 12 wherein said body of metal oxide varistor material has an alphaexponent in excess of l0.

ill

23 An improved electric current collection means comprising first andsecond electrically conductive members adapted for relative motionbetween each other and in close proximity to each other for achievingcurrent transfer therebetween, and a body of metal oxide varistormaterial forming a portion of said first member along a surface which isin close proximity to said second member disposed in conjunction withthe current path between said first and second members for improving thecurrent transfer and, or the commutation of the current. 24. Theimproved current collection means set forth in claim 23 wherein saidbody of metal oxide varistor material comprises a first body retainedalong the trailing end surface of said first member. 25. The improvedcurrent collection means set forth in claim 24 and further comprising asecond body of metal oxide varistor material retained along the leadingend surface of said first member. 26. The improved current collectionmeans set forth in claim 24 wherein said first body of metal oxidevaristor material is of a generally rectangular shape. 27. The improvedcurrent collection means set forth in claim 23 and further comprisingmeans for retaining said first member in a desired position relative tosaid second member, a portion of said retaining means in closestproximity to said second member fabricated of said body of metal oxidevaristor material. 28. The improved current collection means set forthin claim 23 and further comprising means for retaining said first memberin a desired position relative to said second member, said body of metaloxide varistor material rigidly attached to an end surface of saidretaining means along the trailing edge thereof and extending beyond thesurface thereof in closest proximity to said second member. 29. Theimproved current collection means set forth in claim 23 wherein saidbody of metal oxide varistor material has an alpha exponent in excess of10.

2. The improved brush-brush holder assembly set forth in claim 1 whereinsaid body of metal oxide varistor material is a first body forming aportion of said brush along the trailing edge thereof in contact withthe commutator and a second body forming a portion of said brush alongthe leading edge thereof in contact with the commutator.
 3. The improvedbrush-brush holder assembly set forth in claim 1 wherein said body ofmetal oxide varistor material comprises a first body spring loadedindependently of the spring loading of said brush and retained betweenthe trailing end surface of said brush and an adjacent inner end surfaceof said brush holder, the indepenent spring loading of said brush andsaid body of metal oxide varistor material providing compensation fordifferences in wear rate of the metal oxide varistor material and brushmaterial due to contact with the rotating commutator.
 4. The improvedbrush-brush holder assembly set forth in claim 3 and further comprisinga second body of metal oxide varistor mateiral spring loadedindependently of the spring loading of said brush and retained betweenthe leading end surface of said brush and an adjacent inner end surfaceof said brush holder.
 5. The improved brush-brush holder assembly setforth in claim 3 wherein said first body of metal oxide varistormaterial is of a generally rectangular shape.
 6. The improvedbrush-brush holder assembly set forth in claim 1 wherein said brushholder is fabricated of said body of metal oxide varistor material, andincluding spring means in communication with said brush holder formaintaining a contact pressure of the bottom surface of said brushholder upon the commutator.
 7. The improved brush-brush holder assemblyset forth in claim 1 wherein A bottom portion of said brush holder isfabricated of said body of metal oxide varistor material, and includingspring means in communication with said brush holder for maintaining acontact pressure of the bottom surface of said brush holder upon thecommutator.
 8. The improved brush-brush holder assembly set forth inclaim 1 wherein a bottom portion of said brush holder along the trailingend thereof is fabricated of said body of metal oxide varistor materialand extends beyond the bottom surface of the remainder of said brushholder, and including spring means in communication with said brushholder for maintaining a contact pressure of the bottom surface of saidbody of metal oxide varistor material upon the commutator.
 9. Theimproved brush-brush holder assembly set forth in claim 1 wherein saidbody of metal oxide varistor material is rigidly attached to an endsurface of said brush holder along the trailing edge thereof and extendsbeyond the bottom surface of said brush holder, and including springmeans in communication with said brush holder for maintaining a contactpressure of the bottom surface of said body of metal oxide varistormaterial upon the commutator.
 10. The improved brush-brush holderassembly set forth in claim 1 wherein said body of metal oxide varistormaterial is connected across two side walls of said brush holder to formthe trailing end thereof, said body of metal oxide varistor materialextending beyond the bottom surface of said brush holder, and includingspring means in communication with said brush holder for maintaining acontact pressure of the bottom surface of said body of metal oxidevaristor material upon the commutator.
 11. The improved brush-brushholder assembly set forth in claim 1 wherein said body of metal oxidevaristor material has an alpha exponent in excess of
 10. 12. An improvedspark-suppressing brush-brush holder assembly for brush-type electricalmachines comprising a spring-loaded brush, a brush holder for retainingsaid brush in a desired position relative to one or more electricalconductors against which said brush maintains a contact pressuredetermined by the spring-loading thereof, and a body of metal oxidevaristor material forming a portion of said brush along the trailingedge thereof in contact with one of said conductors disposed in thecurrent path between said brush and said conductors when the brush andsaid conductors part during relative motion between said conductors andbrush-brush holder assembly, the body of metal oxide varistor mateiralhaving nonlinear resistance characteristics which limit the voltagedeveloped across the brush and each conductor when they first part. 13.The improved brush-brush holder assembly set forth in claim 12 whereinsaid body of metal oxide varistor material is a first body forming aportion of said brush along the trailing edge thereof and a second bodyforming a portion of said brush along the leading edge thereof.
 14. Theimproved brush-brush holder assembly set forth in claim 12 wherein saidbody of metal oxide varistor material comprises a first body springloaded independently of the spring loading of said brush and retainedbetween the trailing end surface of said brush and an adjacent inner endsurface of said brush holder, the independent spring loading of saidbrush and said body of metal oxide varistor material providingcompensation for differences in wear rate of the metal oxide varistormaterial and brush material due to the relative motion between saidconductors and brush-brush holder assembly.
 15. The improved brush-brushholder assembly set forth in claim 14 and further comprising a secondbody of metal oxide varistor material spring loaded independently of thespring loading of said brush and retained between the leading endsurface of said brush and an adjacent inner end surface of said brushholder.
 16. The improved brush-brush holder assembly set forth iN claim14 wherein said first body of metal oxide varistor material is of agenerally rectangular shape.
 17. The improved brush-brush assembly setforth in claim 12 wherein said brush holder is fabricated of said bodyof metal oxide varistor material, and including spring means incommunication with said brush holder for maintaining a desired contactpressure of the bottom surface of said brush holder upon the conductors.18. The improved brush-brush holder assembly set forth in claim 12wherein a bottom portion of said brush holder is fabricated of said bodyof metal oxide varistor material, and including spring means incommunication with said brush holder for maintaining a contact pressureof the bottom surface of said brush holder upon the conductors.
 19. Theimrpoved brush-brush holder assembly set forth in claim 12 wherein abottom portion of said brush holder along the trailing end thereof isfabricated of said body of metal oxide varistor material and extendsbeyond the bottom surface of the remainder of said brush holder, andspring means in communication with said brush holder for maintaining acontact pressure of the bottom surface of said body of metal oxidevaristor material upon the conductors.
 20. The improved brush-brushholder assembly set forth in claim 12 wherein said body of metal oxidevaristor material is rigidly attached to an end surface of said brushholder along the trailing edge thereof and extends beyond the bottomsurface of said brush holder, and spring means in communication withsaid brush holder for maintaining a contact pressure of the bottomsurface of said body of metal oxide varistor material upon theconductors.
 21. The improved brush-brush holder assembly set forth inclaim 12 wherein said body of metal oxide varistor material is connectedacross two side walls of said brush holder to form the trailing endthereof, said body of metal oxide varistor material extending beyond thebottom surface of said brush holder, and spring means in communicationwith said brush holder for maintaining a contact pressure of the bottomsurface of said body of metal oxide varistor material upon theconductors.
 22. The improved brush-brush holder assembly set forth inclaim 12 wherein said body of metal oxide varistor material has an alphaexponent in excess of
 10. 23 An improved electric current collectionmeans comprising first and second electrically conductive membersadapted for relative motion between each other and in close proximity toeach other for achieving current transfer therebetween, and a body ofmetal oxide varistor material forming a portion of said first memberalong a surface which is in close proximity to said second memberdisposed in conjunction with the current path between said first andsecond members for improving the current transfer and, or thecommutation of the current.
 24. The improved current collection meansset forth in claim 23 wherein said body of metal oxide varistor materialcomprises a first body retained along the trailing end surface of saidfirst member.
 25. The improved current collection means set forth inclaim 24 and further comprising a second body of metal oxide varistormaterial retained along the leading end surface of said first member.26. The improved current collection means set forth in claim 24 whereinsaid first body of metal oxide varistor material is of a generallyrectangular shape.
 27. The improved current collection means set forthin claim 23 and further comprising means for retaining said first memberin a desired position relative to said second member, a portion of saidretaining means in closest proximity to said second member fabricated ofsaid body of metal oxide varistor material.
 28. The improved currentcollection means set forth in claim 23 and further comprising means forretaining said first member in a desired position relative to saidsecond member, said body of metal oxide varistor material rigidlyattached to an end surface of said retaining means along the trailingedge thereof and extending beyond the surface thereof in closestproximity to said second member.
 29. The improved current collectionmeans set forth in claim 23 wherein said body of metal oxide varistormaterial has an alpha exponent in excess of 10.